When I was looking at getting my RC, these are the things I looked at.
1. Look at the under side, specifically for any dents on the skid plates from being abused off road.
2. Look at the axles for dents, scratches, or damage from hitting anything (again, hitting something off road).
3. Have someone start it while you're outside looking at the exhaust pipe and watch for smoke (bad valve seals, etc).
4. Take it for a test drive, take notice of any "odd" noises or rattles that might mean broken/loose parts or nuts/bolts
5. Look for any abnormal leaks
6. Pull the dip stick on the motor and trany and smell it to make sure they're O.K.
7. Make sure all the gauges work.
After you get it home, put it on jackstands and take the tires off to get a look at the brakes, and make sure the seals, wheel cylinders, calibers are not leaking and that the rubber and linning are in good shape.
When I was looking, you wouldn't believe some on the crap I saw. On one truck, the skid plate was dented so bad, it pushed the trany into the floorboard. Another had a leaking front axle from 1/2" wide gouge from what I think he drove over a concrete barrier or something.